With the development of information society, there is a strong demand for development of magnetic recording media that allow high density recording, and the in-depth research and development in the recent few years has realized media with a remarkable high density. However, as the information society is expected to evolve further in the future, there is no technological prospect that market needs will be satisfied in ten or twenty years ahead. A large factor that causes this technological impasse is the following problem of the current magnetic recording media.
Thin films for the current magnetic recording media are alloy thin films based on CoCr. In these thin films, magnetic separation of micro-regions responsible for magnetism is insufficient, so that relatively large magnetic clusters that are coupled magnetically are formed. The size is in the submicron to micron order. In view of the fact that the minimum bit size in the current magnetic recording technology is in the submicron order, which is about the same level as the size of the magnetic clusters, the limit has almost been reached in terms of recording resolution. In order to overcome this limit of the current technology, it is necessary to magnetically insulate magnetic particles in a recording medium and reduce the size of the magnetic clusters.
As a breakthrough to solve this problem, a granular medium was proposed. The granular medium has a structure in which magnetic particles are precipitated in a non-magnetic matrix such as an oxide, and the magnetic particles are magnetically insulated almost completely by the non-magnetic substance interposed therebetween. Therefore, each particle is the minimum magnetization unit and it is possible to record in a high density to an extent of at least this size. In recent years, it has been reported that in the granular medium in which magnetic particles are dispersed and precipitated in a SiO2 non-magnetic matrix, high density recording is possible, and noise can be reduced by preventing large magnetic clusters from being formed.
As described above, the granular medium is a very promising possibility as the next generation super high-density recording medium, but it has a serious problem such as thermal disturbance in the recorded state. In general, a magnetic substance exhibits crystalline magnetic anisotropy that reflects the spatial symmetry of the crystal lattice. For example, in cobalt having a hexagonal close-packed arrangement, the magnetic energy is lowest when the spin is oriented to the direction of the crystal principal axis (c axis), and the magnetic energy is increased when it is displaced from that direction. The magnetic energy is largest when the spin is oriented to the direction orthogonal to the c axis. In other words, if there is no force from the external field, the spin is oriented to either one of the two directions of the c axis direction.
The utilization of binary information of this spin orientation is the basis of magnetic recording. When one magnetic particle is focused on, the total magnetic anisotropic energy thereof is a result obtained by multiplying the volume of the particle by the magnetic anisotropic constant that is determined inherently by the substance. This energy dominates the degree of spin constraint to a stable direction, and leads to the preservation of a recorded state. If the volume of a magnetic particle is extremely small, and the magnetic anisotropic energy is about in the same level as the thermal energy, then thermal disturbance constantly swings the orientation of the spin (i.e., recorded state), so that the recorded state cannot be kept stable.
In the granular medium, very fine particles are almost completely isolated by a non-magnetic substance, this thermal disturbance becomes a very serious problem. Therefore, the granular medium has problems in terms of the thermal stability or the long-term preservability of recorded information, and thus the granular medium is regarded as being difficult to put into practice. In order to solve these problems, it is necessary to essentially increase the anisotropic energy of the magnetic substance, and for this purpose, it has been proposed to use an alloy having a high crystalline magnetic anisotropy for the magnetic recording medium.
Magnetic materials such as FePt, FePd and CoPt exhibit large uniaxial crystalline magnetic anisotropy (FePt: 7×107 erg/cc, saturation magnetization 1140 emu/cc, CoPt: 5×107 erg/cc, saturation magnetization 800 emu/cc) in the CuAu-I type L10 ordered phase (γ1 phase, face-centered tetragonal). The anisotropic energy is at least 20 times larger than that of conventional CoCr alloy based magnetic recording materials, and these materials are gaining attention as magnetic materials that can solve the problem of thermal disturbance in high density magnetic recording as described above.
In recent years, in the field of thin film media, there are a large number of studies regarding FePt, and it is reported that when a thin film of FePt is subjected to a heat treatment at a temperature of 600 to 700° C. in a vacuum, then a transition to the L10 ordered phase occurs, a large crystal magnetic anisotropy is exhibited, and the coercive force reaches 10,000 Oe.
IBM Corporation of the United States released the studies regarding the synthesis of Fe52Pt48 fine particles by a chemical approach in March 2000 (see Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices, S. Sun et al., Science, Vol. 287, p1989-1992 (2000)). The FePt fine particles are monodisperse, has a diameter of about 4 nm and almost no particle size distribution, and are self-aligned in the closest packing manner. Also in the chemical production of IBM, a heat treatment at 550 to 600° C. is necessary to cause a transition to the L10 ordered phase in the FePt fine particles. After the heat treatment, the anisotropic energy of the FePt fine particles is estimated to be 5.9×107 erg/cc, and the KuV/kBT value, which indicates the thermal fluctuation stability, is estimated to be 48.
In the method for synthesizing FePt fine particles of IBM, an iron carbonyl complex (Fe(CO)5) and a platinum complex (Pt(C5H7O2)2) are used. The platinum ions are reduced to metal platinum with 1,2-hexadecane diol, and the iron carbonyl complex is subjected to thermal decomposition to produce iron. As organic protective agents, oleylamine and oleic acid are added to dioctyl ether, which is a synthetic reaction system solvent, and FePt fine particles are synthesized from the atoms of the two metals. These organic protective agents (oleylamine and oleic acid) have a very important role in the synthesis of the FePt fine particles, and thus the particle size of the FePt fine particles are controlled, and the FePt fine particles are separated with a predetermined interval by their molecular steric hindrance for magnetic insulation of the fine particles from each other.
It can be said that the FePt fine particles of IBM showed an ideal form of a material for high density magnetic recording. The fine particles are completely insulated magnetically by the organic protective agent, and the minimum unit of magnetization is as small as 4 nm and uniform. The FePt fine particles dispersed with the organic protective agent are soluble in a solvent and thus can be expected to be used for a hard disk or an application-type medium for high density recording by applying the solvent with the FePt fine particles to a non-magnetic substrate.
However, in the method for synthesizing the FePt fine particles of IBM, iron pentacarbonyl (Fe(CO)5) is used as the source of iron. Fe(CO)5 is a highly toxic and flammable liquid, and produces carbon monoxide which is harmful to the human body in the process of the reaction, as shown in the following reaction formula:Fe(CO)5→Fe+5CO
The synthesis of toxic and flammable materials is hazardous, and the synthesis method that may produce harmful substances is a backward movement to the current trend from the industrial and environmental preservation point of view. In addition, iron pentacarbonyl is more expensive than iron sulfate (FeSO4-7H2O), which is a common iron source. In order to solve this problem, as the source of Fe or Co, their sulfates, chloride salts, phosphates, and sulfonates are used. These compounds do not produce harmful carbon monoxide in the synthesis process, and are less expensive than iron pentacarbonyl. However, in this method, a composition (Fe content: 40 to 50 at %) of a transition metal that is necessary for a phase transition to the L10 ordered phase by a heat treatment cannot be obtained, and the compositions of Fe and Co are about 15 to 25%. Moreover, there is a method as follows: The pH of a reaction solution is adjusted to 9 to 12, and a hydroxide colloid comprising a transition metal and a noble metal is prepared, so that the difference in the oxidation-reduction potential between the transition metal and the noble metal is eliminated, and thus metal alloy fine particles made of the transition metal and the noble metal that are zelovalent in the oxidization state are produced. However, also in this method, the compositions of Fe and Co are about 39%. Furthermore, when they are synthesized in an alkaline atmosphere, the particle size becomes as small as 1 to 1.5 nm, and thermal demagnetization may occur.
Therefore, it is an object of the present invention to provide a novel method for synthesizing monodisperse metal alloy fine particles having a uniform particle size that constitute the basis of the CuAu-I type L10 ordered phase and are made of a transition metal and a noble metal safely and inexpensively.